Magnetic random access memory (MRAM) is a type of memory device containing an array of MRAM cells that store data using their resistance values instead of electronic charges. Generally, each MRAM cell includes a magnetic tunnel junction (MTJ) structure. The MTJ structure typically includes a stack of magnetic layers having a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric, e.g., an insulating tunneling layer. A top electrode and a bottom electrode are utilized to sandwich the MTJ structure so electric current may flow between the top and the bottom electrode.
One type of MRAM cell is spin-transfer-torque magnetic random access memory (STT-MRAM). In such a fabrication process flow, a stable magnetic tunnel junction (MTJ) stack is utilized to sustain high temperature backend thermal processing while still producing a high tunnel magnetoresistance (TMR) ratio. The MTJ stack often utilizes a buffer layer to improve adhesion and the seeding of the subsequent layers. The MTJ stack also includes a synthetic ferrimagnet (SyF) coupling layer to couple the first pinning layer and the second pinning layer in an antiparallel manner. A capping layer is utilized on top of the MTJ stack which protects the stack from corrosion and also acts as a etch stop layer for hard mask etching. The capping layer, which interfaces with a magnetic storage layer of the MTJ, is utilized to create a sufficient perpendicular magnetic anisotropy (PMA) to provide for a data retention energy barrier.
Conventional capping layers at the interface with the magnetic storage layer utilize boron to maintain a sufficient PMA. However, after high temperature thermal processing, the boron diffuses away from the interface and weakens the magnetic storage layer PMA. Such conventional capping layers utilize a magnesium oxide (MgO) material, however, to maintain a suitable PMA a thicker MgO material is utilized. The thicker MgO material increases surface roughness of the interface and reduces TMR.
Therefore, there is a need in the art for improved methods for fabricating MTJ structures for STT-MRAM applications. There is also a need for improved MTJ stacks that are able to sustain high temperature thermal processing while preserving high PMA.